1. Field of the Invention
This invention pertains to methods and devices for maintaining a desired therapeutic drug effect over a prolonged therapy period. In particular, the invention is directed to methods and devices that provide drug release within the gastrointestinal tract at an ascending release rate over an extended time period. In this manner, drug is released at an ascending rate during a portion of the drug administration period sufficient to maintain a desired therapeutic drug effect throughout a prolonged therapy period.
2. Description of the Related Art
To produce its pharmacological effects, a drug must be made available in appropriate concentrations at its site of action within the body. This availability is affected by numerous factors including the quantity of the drug administered, the extent and rate of its absorption from its administration site, its distribution, binding or localization within tissues, its biotransformation and its excretion. One commonly-used indicator of drug availability is the concentration of drug that is obtained within the blood or plasma, or other appropriate body fluid or tissue, of a patient following administration of the drug. For convenience, this concentration may be referred to as “plasma drug concentration” hereinafter which is intended to be inclusive of drug concentration measured in any appropriate body fluid or tissue. Plasma drug concentration measurements provide very useful information including, for example, comparative information with regard to different drug dosage forms and/or different drug administration routes. In addition, for many drugs, various drug effects including both desired pharmacological effects, i.e., therapeutic drug effects, and undesired pharmacological effects, i.e., side effects, have been correlated with specific plasma drug concentrations or ranges of plasma drug concentrations.
For orally administered drug dosage forms, absorption occurs within the gastrointestinal (“g.i.”) tract and is affected by many factors including the physicochemical properties of the local microenvironment, such as surface area, blood flow and membrane characteristics (which vary significantly in the different portions of the g.i. tract), the physicochemical properties of the drug entity, drug concentration, the existence and activity of drug-specific transport mechanisms, etc. One important factor in the rate of absorption of drug administered as an oral dosage form is the rate at which drug is released from the dosage form. Drug release rates for oral dosage forms are typically measured as an in vitro rate of dissolution, i.e., a quantity of drug released from the dosage form per unit time.
Conventional oral dosage forms can be described as “immediate-release” because, generally, essentially the entire dose of drug is released from the dosage form within a very short period, i.e., minutes, following administration. As this bolus of released drug is absorbed, the plasma drug concentration typically rapidly rises to a maximal or peak concentration and subsequently declines as the drug is distributed, bound or localized within tissues, biotransformed and/or excreted. The time period for this decline varies for different drugs and depends on many factors but this time period will be characteristic of a particular drug. Generally, during some portion of the time period in which the plasma drug concentration rises, peaks and declines, the drug provides its therapeutic effects, i.e., the plasma drug concentration achieves or exceeds an effective concentration. Moreover, at some point during this time period, the therapeutic effects disappear, i.e., when the plasma drug concentration declines to a level that is below an effective concentration. In addition, often, during a portion of this time surrounding the time the peak concentration is attained, i.e., when the plasma drug concentration is in its highest range, undesired side effects may become apparent.
In view of the above, it will be appreciated that continued drug effectiveness occurs during the time period when the plasma drug concentration is within the effective plasma drug concentration range. Because the plasma drug concentration declines over time, however, multiple doses of the immediate-release drug dosage form must be administered at appropriate intervals to ensure that the plasma drug concentration remains in or, again, rises to, the effective concentration range. At the same time, however, there is a need to avoid or minimize plasma drug concentrations that rise to, and/or that remain for too long within, the higher ranges where side effects become apparent. Accordingly, for many drugs, multiple, separate doses of the immediate-release dosage form must be administered at appropriate intervals to maintain a satisfactory balance of desired and undesired pharmacological effects over a prolonged therapy period.
One focus of efforts to improve drug therapy has been directed to providing non-immediate-release oral drug dosage forms that affect absorption of the drug primarily by altering the release rate of the drug from the dosage form. Examples of such non-immediate-release delivery systems include delayed-release and sustained-release systems. Sustained-release dosage forms generally release drug for an extended time period compared to an immediate-release dosage form. There are many approaches to achieving sustained release of drugs from oral dosage forms known in the art. These different approaches include, for example, diffusion systems such as reservoir devices and matrix devices, dissolution systems such as encapsulated dissolution systems (including, for example, “tiny time pills”) and matrix dissolution systems, combination diffusion/dissolution systems, osmotic systems and ion-exchange resin systems as described in Remington's Pharmaceutical Sciences, 1990 ed., pp. 1682-1685.
It is believed to be particularly desirable to provide sustained-release oral dosage forms that provide drug release at a substantially constant release rate over an extended time period. In this manner, for many drugs, the plasma drug concentration initially ascends for a short period of time as drug release begins and then remains substantially constant over an extended time period as drug release continues at a constant rate. For many drugs, this substantially constant plasma drug concentration correlates with substantially constant drug effectiveness over a prolonged therapy period. In addition, because an initial relatively high peak plasma drug concentration is avoided, side effects may be less of a problem. Accordingly, advantages of constant-release dosage forms include decreasing the number of doses of a drug that need to be administered over time and providing a better balance of desired and undesired pharmacological effects of the drug.
Osmotic dosage forms, in particular, have been notably successful at providing constant-release of drugs over extended time periods. Osmotic dosage forms, in general, utilize osmotic pressure to generate a driving force for imbibing fluid into a compartment formed, at least in part, by a semipermeable wall that permits free diffusion of fluid but not drug or osmotic agent(s), if present. A substantially constant rate of drug release can be achieved by designing the system to provide a relatively constant osmotic pressure and having suitable exit means for the drug formulation to permit the drug formulation to be released at a rate that corresponds to the rate of fluid imbibed as a result of the relatively constant osmotic pressure. A significant advantage to osmotic systems is that operation is pH-independent and thus continues at the osmotically-determined rate throughout an extended time period even as the dosage form transits the gastrointestinal tract and encounters differing microenvironments having significantly different pH values.
Surprisingly simple but highly effective osmotic devices comprising drug in a mixture with excipients, optionally including osmotically active component(s), within the compartment are known in the art. Although effective for many drugs, the release rate in these devices often declines over time and complete delivery of the drug load may not occur. A more sophisticated type of osmotic device comprises two component layers within the compartment formed by the semipermeable wall. One component layer comprises drug in a mixture with excipients, optionally including osmotically active component(s), that will form a deliverable drug formulation within the compartment and the second component layer comprises osmotically active component(s) but does not contain drug. The osmotically active component(s) in the second component layer typically comprise osmopolymer(s) having relatively large molecular weights and which exhibit “swelling” as fluid is imbibed such that release of these components through the drug formulation exit means does not occur. The second component layer is referred to as a “push” layer since, as fluid is imbibed, the osmopolymer(s) swell and push against the deliverable drug formulation of the first component layer to thereby facilitate release of the drug formulation at a substantially constant rate. The above-described devices are known, for example, from the following US patents, owned by Alza Corporation: U.S. Pat. Nos. 4,327,725; 4,612,008; 4,783,337; and 5,082,668, each of which is incorporated in its entirety by reference herein.
Although constant-release dosage forms have proven effective for many different drug therapies, there are clinical situations where these have not been entirely satisfactory. It has been observed that for some patients being treated with constant-release dosage forms for some conditions or diseases, the therapeutic effectiveness of the drug decreases at time periods before the end of the desired therapy period despite the maintenance of substantially constant drug release that would be expected to provide continued effectiveness. Accordingly, there remains a need to provide methods and devices for maintaining a desired therapeutic drug effect over a desired prolonged therapy period when sustained-release dosage forms that release drug at a substantially constant rate over an extended time period are not satisfactory.